1. Field of the Invention
This invention relates to an objective lens used for microscopes and, in particular, to an objective lens suitable for microscopy of a specimen through a thick plane-parallel plate, such as a cultivating vessel or a liquid crystal display substrate.
2. Description of Related Art
In general microscopy, when a thick plane-parallel plate is interposed between a specimen and an objective lens, spherical aberration will be produced depending on its thickness. For conventional objective lenses, the following two techniques of correcting for spherical aberration have been chiefly adopted.
(1) A lens unit which is movable along the optical axis is disposed in an objective unit, which is corrected for spherical aberration by the movement of the lens unit and which is usually called an objective lens with a rotary ring for fine adjustment of focus. PA0 (2) A removal, transparent plane-parallel member is previously disposed at the front end of an objective lens, and the thickness of the plane-parallel member is changed so that spherical aberration can be corrected depending on the thickness of a plane-parallel plate lying on the specimen side (for example, a technique set forth in Japanese Utility Model Preliminary Publication No. Sho 61-16526).
In either of the techniques mentioned above, it is common practice for design of the objective lens to use the characteristic values of a particular material (optical element) or virtual values as the optical constants (refractive index and Abbe's number) of a thick plane-parallel plate, assumed as a vessel, on the specimen side.
However, where an objective lens produced through either of the above techniques is actually used, if the optical constants of the thick plane-parallel plate employed on the specimen side differ from those assumed in the design of the objective lens, its optimum imaging performance will not be obtained. For example, consider the case where a cultivating vessel is used for observation of cells. Cultivating vessels are available in glass and plastic materials. If an objective lens used in this case is designed on the assumption that tile plane-parallel plate is made of glass, the optimum imaging performance will be secured in the case where the cultivating vessel of glass is employed. Conversely, the use of the cultivating vessel of plastic leads to a degraded contrast of an image available. This is because the dispersion values (usually called Abbe's numbers) of materials of the plane-parallel plates are different from each other. In this way, when the plane-parallel plate assumed in the design of the objective lens is different in material from that in practical use, there is the problem that the above phenomenon is produced and consequently, design performance cannot be brought about (refer to FIGS. 1A and 1B).
It may be considered that an objective lens is designed so that a plurality of plane-parallel plates constructed of different materials is actually used and their refractive indices and Abbe's numbers are set to the respective average values. However, this approach, although reducing the difference in imaging performance between the plane-parallel plates, has the problem that the imaging performance exhibits a compromise state and thus the optimum imaging performance cannot be obtained.